U.S. patent number 4,882,107 [Application Number 07/276,162] was granted by the patent office on 1989-11-21 for mold release coating process and apparatus using a supercritical fluid.
This patent grant is currently assigned to Union Carbide Chemicals and Plastics Company Inc.. Invention is credited to Keith D. Cavender, Edmond J. Derderian, Eugene L. Jarrett, Kenneth A. Nielsen.
United States Patent |
4,882,107 |
Cavender , et al. |
November 21, 1989 |
Mold release coating process and apparatus using a supercritical
fluid
Abstract
A process which comprises (i) the generation of release surfaces
by application to predetermined areas of a solid surface of a
solution, suspension or dispersion of a release agent and a
supercritical fluid that vaporizes from the release agent, (ii) the
deposition of a mass onto the release surface containing the
release agent, and (iii) the separation of the mass or a product
derived from the mass from such surface covered by the release
agent. Novel apparatus for carrying out the process are
described.
Inventors: |
Cavender; Keith D. (Charleston,
WV), Derderian; Edmond J. (Charleston, WV), Jarrett;
Eugene L. (St. Albans, WV), Nielsen; Kenneth A.
(Charleston, WV) |
Assignee: |
Union Carbide Chemicals and
Plastics Company Inc. (Danbury, CT)
|
Family
ID: |
23055450 |
Appl.
No.: |
07/276,162 |
Filed: |
November 23, 1988 |
Current U.S.
Class: |
264/51;
264/DIG.83; 264/175; 264/310; 264/328.1; 264/328.2; 264/523;
425/817C; 426/515; 264/54; 264/299; 264/319; 264/338; 425/90;
425/817R |
Current CPC
Class: |
B29C
33/58 (20130101); B29C 33/60 (20130101); B05D
1/025 (20130101); Y10S 264/83 (20130101); B05D
2401/90 (20130101) |
Current International
Class: |
B29C
33/60 (20060101); B29C 33/58 (20060101); B29C
33/56 (20060101); B05D 1/02 (20060101); B29C
067/22 (); B29C 033/58 (); B29C 039/36 (); B29C
045/40 () |
Field of
Search: |
;264/51,338,DIG.83,523,319,299,175,310,328.1,328.2,54
;425/90,817R,817C ;426/512,515 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Long, George E. "Spraying Theory and Practice" in Chemical
Engineering, Mar. 13, 1978, pp. 73-77. .
CRC Handbook of Chemistry & Physics, 67th Edition, 1986-1987,
Boca Raton, Fla., CRC Press, Inc., pp. R-62 to R-64. .
Kirk-Othamer Encyclopedia or Chemical Technology, Third Edition,
vol. 6, New York, John Wiley & Sons, c 1979, A
Wiley-Interscience Publication, pp. 386-426. .
Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition,
vol. 21, New York, John Wiley & Sons, c 1983, A
Wiley-Interscience Publication, pp. 466-483..
|
Primary Examiner: Anderson; Philip
Attorney, Agent or Firm: Leuzzi; Paul W.
Claims
We claim:
1. A process which comprises the steps of (i) forming a release
surface by spray coating an area of a solid surface with a release
agent obtained from a release agent formulation containing a
solution, suspension or dispersion of a release agent and a
supercritical fluid, (ii) depositing a mass of material onto the
release surface, and (iii) separating the mass of material or a
product derived from the mass of material, from the release
surface.
2. A process which comprises (i) generating release surfaces by
spray coating a release agent obtained from a solution, suspension
or dispersion of a release agent and a supercritical fluid that
vaporizes from the release agent from a high pressure zone through
an orifice to a low pressure zone outside of the orifice, to form a
spray of release agent particles which are deposited onto
predetermined areas of a solid surface, (ii) depositing a mass onto
the release surface containing the release agent, and (iii)
separating the mass or a product derived from the mass from such
surface which is covered by the release agent.
3. The process of claim 2 wherein the deposition of the release
agent on the predetermined areas is as an essentially uniform film
covering the predetermined area.
4. The process of claim 2 wherein the spray composition is heated
prior to atomization.
5. The process of claim 4 wherein the minimum spray temperature is
about 31.degree. C. and the maximum temperature is determined by
the thermal stability of the components in the liquid mixture.
6. The process of claim 5 wherein the spray temperature is between
35.degree. C. and 90.degree. C.
7. The process of claim 6 wherein the spray temperature is between
45.degree. C. and 75.degree. C.
8. The process of claim 1 wherein the mass of material is selected
from the group consisting of thermoplastic resins thermosetting,
resins, elastomers, food preparations and protective coatings.
9. The process of claim 1 wherein the mass that is deposited is a
hardenable material.
10. The process of claim 1 wherein the spray coating involves the
uniform deposition of a thin layer of the release agent over a
predetermined surface area.
11. The process of claim 10 wherein migration of the release agent
from such area is avoided.
12. The process of claim 1 wherein migration of the release agent
into the subsequently applied material is avoided.
13. The process of claim 1 wherein the mass of material or a
product derived from the mass of material separated from the
surface are obtained essentially free of surface defects derived
from interaction with the release agent.
14. The process of claim 1 wherein the deposition (ii) is effected
by coating the area and the separation (iii) of the coating is
effected by brushing the coated surface until the coating is
removed.
15. The process of claim 1 comprising the molding of thermosetting
resins and thermoplastics to form shaped articles, in which the
interior molding surface of the mold in which the thermosetting
resins and thermoplastics are to be molded are sprayed with a
release agent obtained from a solution, suspension or dispersion
comprising a release agent and a supercritical fluid that vaporizes
from the sprayed release agent, the thermosetting resin or
thermoplastic are thereafter supplied to the mold to contact the
interior molding surface and molded therein to effect the shape
conferred by the mold, and then the molded shaped article is
removed from the mold.
16. The process of claim 1 wherein the supercritical fluid is
carbon dioxide.
17. The process of claim 2 wherein the supercritical fluid is
carbond dioxide.
18. The process of claim 3 wherein the supercritical fluid is
carbon dioxide.
19. The process of claim 8 wherein the supercritical fluid is
carbon dioxide.
20. The process of claim 9 wherein the supercritical fluid is
carbon dioxide.
21. The process of claim 10 wherein the supercritical fluid is
carbon dioxide.
22. The process of claim 11 wherein the supercritical fluid is
carbon dioxide.
23. The process of claim 12 wherein the supercritical fluid is
carbon dioxide.
24. The process of claim 13 wherein the supercritical fluid is
carbon dioxide.
25. The process of claim 14 wherein the supercritical fluid is
carbon dioxide.
26. The process of claim 16 wherein the supercritical fluid is
carbon dioxide.
27. The process of claim 16 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
28. The process of claim 17 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
29. The process of claim 18 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
30. The process of claim 19 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
31. The process of claim 20 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
32. The process of claim 24 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
33. The process of claim 22 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
34. The process of claim 23 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
35. The process of claim 24 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
36. The process of claim 25 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
37. The process of claim 26 wherein the formulation contains a
reduced amount of an active solvent for the release agent.
38. The process comprising the steps of:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one release agent capable of forming a thin layer or
coating on a mold surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount of an active solvent or
solvent(s) capable of dissolving, suspending or dispersing the
release agent;
b. spraying said fluid mixture onto a mold surface to form a thin
layer of the release agent thereon;
c. introducing a molding composition to the mold surfaces
containing the thin layer of release agent thereon and molding the
composition in the mold to form a molded product; and
d. separating the molded product from the mold.
39. The process of claim 38 wherein molding is effected by one of
reaction injection molding (RIM), injection molding, compression
molding, bulk molding, transfer molding, cast molding, spin cast
molding, casting, vacuum forming, blow molding, calendar molding,
lamination, molding of foam, and rotational molding.
40. A molding process which comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one wax compound capable of forming a layer on the mold
surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount of an active solvent or
solvent(s) capable of dissolving, suspending or dispersing the wax
compound(s);
b. spraying said fluid mixture onto a mold surface to form a thin
wax layer thereon;
c. introducing a molding composition to the mold surfaces
containing the thin wax layer of release agent thereon and molding
the compositions to form a molded product; and
d. separating the molded product from the mold.
41. A molding process which comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one wax compound capable of forming a layer on the mold
surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount of an active solvent or
solvent(s) capable of dissolving, suspending or dispersing the wax
compound(s);
b. spraying said fluid mixture by electrostatic means onto a mold
surface to form a thin wax layer thereon;
c. introducing a molding composition to the mold surfaces
containing the thin wax layer of release agent thereon and molding
the compositions to form a molded product; and
d. separating the molded product from the mold.
42. The process of claim 40 wherein the molding composition
comprises a composition which forms a polyurethane.
43. The process of claim 41 wherein the molding composition
comprises a composition which forms a polyurethane.
44. The process of claim 42 wherein the polyurethane is a
polyurethane resin.
45. The process of claim 43 wherein the polyurethane is a
polyurethane resin.
46. The process of claim 44 wherein the polyurethane is a
polyurethane foam.
47. The process of claim 45 wherein the polyurethane is a
polyurethane foam.
48. A process for the release preparation of mold surfaces in which
the polymerization of an active hydrogen compound and an isocyanate
compound is carried out to form a molded article conforming to the
mold, wherein the mold surfaces are release prepared prior to said
polymerization by the process which comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one wax compound capable of forming a layer on the mold
surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount of an active solvent or
solvent(s) capable of dissolving, suspending or dispersing the wax
compound(s); and
b. spraying said fluid mixture onto a mold surface to form a wax
layer thereon.
49. The process of claim 44 wherein there is formed a polyurethane
foam on the mold surface containing the wax layer.
50. An apparatus for the deposition of a release agent mixture
comprising a supercritical fluid and a release agent onto a release
surface which comprises:
a. means for forming a supercritical fluid,
b. heating means for reducing the viscosity of a release agent,
c. means for combining the release agent and the supercritical
fluid into a release agent mixture and maintaining the release
agent mixture in the supercritical fluid state, and
d. means for spraying the release agent mixture in the
supercritical state onto a release surface, and
e. a release surface onto which the release agent mixture is
deposited.
Description
RELATED PATENT APPLICATIONS
This application contains subject matter related to U.S. patent
applications Ser. No. 883,156, filed Jul. 8, 1986, abandoned,
133,068, filed Dec. 21, 1987, 218,896, filed Jul. 14, 1988, and
218,910, filed Jul. 14, 1988.
BRIEF DESCRIPTION OF THE INVENTION
A process which comprises (i) the generation of release surfaces by
application to predetermined areas of a solid surface of a
solution, suspension or dispersion of a release agent and a
supercritical fluid that vaporizes from the release agent, (ii) the
deposition of a mass onto the release surface containing the
release agent, and (iii) the separation of the mass or a product
derived from the mass from such surface covered by the release
agent. Novel apparatus for carrying out the process are
described.
BACKGROUND TO THE INVENTION
The use of supercritical fluids as a transport medium for the
manufacture of surface coatings is well known. German patent
application 28 53 066 describes the use of a gas in the
supercritical state as the fluid medium containing the solid or
liquid coating substance in the dissolved form. In particular, the
application addresses the coating of porous bodies with a
protectant or a reactive or nonreactive decorative finish by
immersion of the porous body in the supercritical fluid coupled
with a pressure drop to effect the coating. The most significant
porous bodies are porous catalysts. However, the applicant
characterizes fabrics as porous bodies.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S.
Pat. No. 4,734,451, patented Mar. 29, 1988, describes forming a
supercritical solution which includes a supercritical fluid solvent
and a dissolved solute of a solid material and spraying the
solution to produce a "molecular spray." A "molecular spray" is
defined as a spray "of individual molecules (atoms) or very small
clusters of the solute." The Smith patents are directed to
producing fine films and powders. The films are used as surface
coatings.
The aforementioned related applications are generally concerned
with the formation of coatings utilizing supercritical fluids to
reduce the viscosity of the coatings formulations. These
applications stress the use of carbon dioxide (CO.sub.2) for
generating the supercritical fluid.
U.S. patent application Ser. No. 133,068, filed Dec. 21, 1987, to
Hoy, et al., discloses a process and apparatus for the liquid spray
application of coatings to a substrate and minimizes the use of
environmentally undesirable organic diluents. The process of the
application involves:
(1) forming a liquid mixture in a closed system, said liquid
mixture comprising:
(a) at least one polymeric compound capable of forming a coating on
a substrate; and
(b) at least one supercritical fluid, in at least an amount which
when added to (a) is sufficient to render the viscosity of said
mixture of (a) and (b) to a point suitable for spray applications;
and
(2) spraying said liquid mixture onto a substrate to form a liquid
coating thereon.
The application is also directed to a liquid spray process in which
at least one active organic solvent (c) is admixed with (a) and (b)
above prior to the liquid spray application of the resulting
mixture to a substrate. The preferred supercritical fluid is
supercritical carbon dioxide. The process employs an apparatus in
which the mixture of the components of the liquid spray mixture can
be blended and sprayed onto an appropriate substrate. The apparatus
contains
(1) means for supplying at least one polymeric compound capable of
forming a continuous, adherent coating;
(2) means for supplying at least one active organic solvent;
(3) means for supplying supercritical carbon dioxide fluid;
(4) means for forming a liquid mixture of components supplied from
(1)-(3); and
(5) means for spraying said liquid mixture onto a substrate.
The apparatus may also provide for (6) means for heating any of
said components and/or said liquid mixture of components. U.S.
patent application Ser. No. 133,068 demonstrates the use of
supercritical fluids, such as supercritical carbon dioxide fluid,
as diluents in highly viscous organic solvent borne and/or highly
viscous non-aqueous dispersions coatings compositions to dilute the
compositions to application viscosity required for liquid spray
techniques. They further demonstrate that the method is generally
applicable to all organic solvent borne coatings systems.
Copending U.S. application Ser. No. 218,910, filed Jul. 14, 1988,
is directed to a liquid coatings application process and apparatus
in which supercritical fluids, such as supercritical carbon dioxide
fluid, are used to reduce to application consistency viscous
coatings compositions to allow for their application as liquid
sprays. The coatings compositions are sprayed by passing the
composition under pressure through an orifice into the environment
of the substrate.
In particular, the process of U.S. application Ser. No. 218,910 for
liquid spray application of coatings to a substrate comprises:
(1) forming a liquid mixture in a closed system, said liquid
mixture comprising:
(a) at least one polymeric component capable of forming a coating
on a substrate; and
(b) a solvent component containing at least one supercritical
fluid, in at least an amount which when added to (a) is sufficient
to render the viscosity of said mixture to a point suitable for
spray application; and
(2) spraying said liquid mixture onto a substrate to form a liquid
coating thereon by passing the mixture under pressure through an
orifice into the environment of the substrate to form a liquid
spray.
U.S. application Ser. No. 218,896, filed Jul. 14, 1988, is directed
to a process and apparatus for coating substrates by a liquid spray
in which (1) supercritical fluid, such as supercritical carbon
dioxide fluid, is used as a viscosity reduction diluent for coating
formulations, (2) the mixture of supercritical fluid and coating
formulation is passed under pressure through an orifice into the
environment of the substrate to form the liquid spray, and (3) the
liquid spray is electrically charged by a high electrical voltage
relative to the substrate.
In particular, the process of U.S. application Ser. No. 218,896 for
electrostatic liquid spray application of coatings to a substrate
comprises:
(1) forming a liquid mixture in a closed system, said liquid
mixture comprising:
(a) at least one polymeric component capable of forming a coating
on a substrate; and
(b) a solvent component containing at least one supercritical
fluid, in at least an amount which when added to (a) is sufficient
to render the viscosity of said mixture to a point suitable for
spray application;
(2) spraying said liquid mixture onto a substrate to form a liquid
coating thereon by passing the mixture under pressure through an
orifice into the environment of the substrate to form a liquid
spray; and
(3) electrically charging said liquid spray by a high electrical
voltage relative to the substrate and electric current.
Many applications in industry utilize solid release surfaces. The
function of solid release surfaces is to allow the deposition of a
material onto the surface and to remove it without having the
material stick to the surface. One way of forming a solid release
surface is to deposit a release agent onto the surface and have the
release agent replicate the surface such that any material to be
deposited onto the surface is not seemingly or intended to be
adversely affected by such release agent. The use of release agents
on a solid surface can create considerable problems, some of which
are not well appreciated. For example-
Where the release surface is a hot surface, the presence of the
release agent on the surface creates a thermal gradient from the
surface to the material applied to it. If the release agent is
irregularly applied then the temperature across the surface of the
release agent as applied to the surface will be nonuniform. This
means that the material applied to the surface containing release
agent will experience a variability of thermal effects. There are
very few situations where this variability will not adversely
affect properties of the material.
One problem associated with supplying a release agent to a
replicating surface is the irregular nature of the deposition owing
to the large amount of release agent inevitably used.
Illustratively, conventional spraying of a release agent to a
release surface involves propelling a solution of the release agent
(generally dissolved in a solvent) by a gas under pressure. The
spray comprises droplets of the release agent and the droplets
coalesce on the sprayed surface to form a film of considerable
thickness. If the release surface is a mold, then the release agent
is sprayed into the mold onto the mold surface(s). The material to
be molded is supplied to the mold and the replication of the mold
surface may take place under heat and pressure. Under the operating
conditions, the mold release agent is to act as a barrier that
keeps the molding material from contacting the mold surface. The
mold release agent does this in three ways, as a vapor, a liquid or
solid. It either vaporizes and provides a vapor barrier to the
surface or it liquefies without vaporization if it starts out as a
solid to form a liquid barrier or loses viscosity without
vaporization if it starts out as a liquid to form a liquid barrier
or it is sprayed from a solvent containing solution to the mold,
and the solvent evaporates in the mold to deposit a solid wax film.
In almost all cases, there is a viscosity reduction in the release
agent that allows it to become more uniformly coated across the
mold surface. However, this does not mean that the mold release
agent exists as a uniform vaporous, liquid or solid layer on the
mold surface. If the amount of release agent is excessive at any
place in the mold, the surface of the mold will ultimately be
nonuniformly coated. The heat from the mold surface received by the
material being supplied and being acted upon in the mold is not
going to be uniformly applied to it and this thermal variance can
adversely affect the molded object being produced. Such adverse
effects will typically exist at the surface of the molded
object.
In the case of irregularly shaped molds or cavity molds, there is a
tendency of the sprayed on release agent to pool into thicker
layers owing to gravitational flow to lower surface portions in the
mold. As a result, there is an assured irregularity in the
temperature across the mold surface that is experienced by the
material being molded. This does not mean that portions of the mold
surface are devoid of mold release agent; to the contrary, the
point to be made is that portions of the mold surface have too much
mold release agent.
Even should the release agent be an uniformly applied layer on the
release surface, the layer is relatively thick; sufficiently so
that the layer penetrates the material being applied to the
surface.
For example, in the baking of goods in a baking pan, there are used
release agents supplied to the surface of the baking pan that are
made of vegetable oils. These oils penetrate the baking
formulations such that the skin of the baked goods is essentially
"french fried" by the vegetable oil and the surfaces of the baked
goods have a consistency different from that of the interior of the
goods. Indeed, if the condition were otherwise, one would question
that the goods were properly baked.
Some plastics possess crystalline and amorphous components.
Penetrating release agents can attack either phase such that the
surface of the molded piece is different from its interior.
Many plastics that are molded are used for food applications where
the food contacts the plastic. Of serious concern is the presence
of mold release agents that adhere to the surface of the resulting
molded plastic part that can adversely affect the performance of
the molded part in one or more aspects of the use of the plastic.
For example, even a thin layer of mold release agent on the surface
of the plastic part has to be removed from the part or else it will
contribute either a taste or texture factor to the food in contact
with the part.
Even when the conventionally sprayed release agent is an uniformly
applied layer on the release surface, the layer is relatively
thick; sufficiently so that the thick layer precludes the use of
release agents in masking portions of a surface for subsequent
application of a coating to the surface.
There are many industrial applications where coating is limited to
certain portions of a surface so that the uncoated surface can be
subsequently used in another manner. For example, portions of a
metal surface may be painted first and another portion left
unpainted so that is can be used to effect bonding between
surfaces. Illustrations are painted automobile or aircraft parts
being adhesively bonded to other parts. In the case of solid state
electronic circuits, portions of a dielectric surface are first
masked before applying the electronic circuit. A problem that
exists in such techniques is that the coating or masking cannot be
applied in an industrial high volume production environment so that
the uncoated portion occupies the minimal area on the surface for
the subsequent application involved. Coating materials have a
tendency to run or migrate therefore to assure that the
coating-free surface remains coating-free, more of coating-free
surface is allocated than is necessary for the subsequent
application in which it is a functional surface. This is more of a
problem where the surface is to be dip coated. It is most difficult
to allocate a coating-free surface by the dipping process. It would
be desirable to form coating-free portions on a surface which is to
be otherwise dip coated and not have the release agent provided on
the coating-free portions adversely affect the portion of the
surface that is to be dip coated.
It would be desirable to be able to pretreat the surface to be
coated with a coating release agent at the portions of the surface
that ultimately are to remain uncoated, and then apply the finish
coating or masking to the whole surface, including that surface
portion containing the coating release agent, and complete the
coating or masking activity, such as curing or drying the coating
or masking. After the activity is over, the surface where the
coating release agent had been applied can be brushed to remove the
unbonded portion of the coating or mask to leave a surface
containing the coating release agent.
The virtue of such a masking procedure resides in the ability to
minimize the size of the uncoated (coating-free) portion of the
surface. Such minimizes the presence of uncoated and unbonded areas
on the surface.
The technique would only be effective if the release agent does not
migrate prior to or during the coating operation and is readily
removable from the substrate. Owing in large part to the excessive
amount of release agent that would be supplied to the surface by
conventional techniques, migration of the release agent during some
phase of the coating operation would occur. This would increase the
coating-free portion of the surface in an uncontrolled manner.
The technique would also require that the amount of release agent
on the coating-free portion of the surface be readily removeable
from the surface so as to avoid interfering with subsequent
utilization of that coating-free surface.
Recognizing that release agents are high boiling or high melting
materials with a high viscosity at ambient temperature and pressure
conditions, in order to apply them to the release surface means
that their viscosity has to be reduced at the moment they are
applied to the surface. This has meant that the release agents had
to be cut with solvents. Assuming that the solvents are
toxicologically safe, their use introduces an environmental
problem. When they are vaporized, they enter the atmosphere and are
believed to contribute to smog formation. For example, hydrocarbon
solvents have been widely employed as solvents for mold release
agents. Enough concern exists that they represent an environmental
problem because of their contribution to smog formation that
water-based mold release formulations have been developed in order
to eliminate these organic solvent emissions. However, the
performance of these water-based compositions is significantly
deficient relative to that of hydrocarbon-based materials
because-
.DELTA. they fail to provide as good release properties;
.DELTA. they create a water disposal problem; and
.DELTA. they may adversely affect the temperature of the surface
being treated.
A novel system has been discovered for the application of a release
agent to a surface over which another material is to be deposited
and then removed. This system provides the ability to uniformly
apply a release agent to a release surface and provides one or more
of the following advantages:
The use of organic solvents can be eliminated or minimized.
The concentration of release agent on the release surface can be
materially reduced.
Solid release agents can be used where liquids had previously been
employed because the release agent can be uniformly deposited as
small particles on the release surface.
The release agent can be supplied as a liquid from a sprayhead of a
spray device and immediately upon clearance from the sprayhead, the
spray exists as a fine mist of particles, each particle having a
much greater viscosity than that of the liquid from which they are
derived.
The release agent may be applied to less of the surface than the
other material is applied to, so that the other material covers
surface containing the release agent and surface which is free of
the release agent. The other material is removed from surface
containing the release agent.
The release agent may be applied using standard spray
technology.
THE INVENTION
The invention relates to a process which comprises (i) the
generation of release surfaces by spray coating predetermined areas
of a solid surface with a release agent obtained from a solution,
suspension or dispersion of a release agent and a supercritical
fluid that vaporizes from the release agent, (ii) the deposition of
a mass onto the release surface containing the release agent, and
(iii) the separation of the mass or a product derived from the mass
from such surface which is covered by the release agent.
More particularly, the invention relates to a process which
comprises (i) generating release surfaces by spray coating a
release agent obtained from a solution, suspension or dispersion of
a release agent and a supercritical fluid that vaporizes from the
release agent from a high pressure zone through an orifice to a low
pressure zone outside of the orifice, to form a spray of release
agent particles which are deposited onto predetermined areas of a
solid surface, preferably as an essentially uniform film covering
the predetermined areas, (ii) depositing a mass onto the release
surface containing the release agent, and (iii) separating the mass
or a product derived from the mass from such surface which is
covered by the release agent.
The invention has broad industrial applications and includes such
arts as molding plastics, resins and elastomers, baking food
products and coating surfaces, all of which utilize a release
surface to prevent the adhesion of plastics, resins, food products
and coatings to the substrate to which they are supplied or
applied, as the case may be. Broadly speaking, the invention has
application wherever a hardenable material is deposited onto a
surface area to which it is not desired to be bonded but of its
nature and that of the surface, it hardens on the surface and
adheres to it sufficiently that there is difficulty in cleanly
removing all of the deposition from the surface. The invention
provides a method of avoiding such adherence.
The invention embraces a process of uniformly depositing a thin
layer of release agent over a predetermined surface in such a
manner that migration of the release agent from such area is
minimized, if not altogether eliminated. The invention also
embraces the deposition of such a thin layer of the release agent
over the predetermined surface that migration of the release agent
into the subsequently applied material is significantly minimized,
preferably essentially avoided. As a result, molded or baked
objects are obtainable essentially free of surface effects derived
from interaction between a release agent and the objects, thereby
making the objects more homogeneous throughout their structures. In
addition, such limited amounts of release agent are provided on a
surface by the process of the invention that little, if any,
release agent flows into adjacent surface area, even when coated
over by a liquid containing a solvent, such as paints, lacquers,
inks, and the like.
Because the amount of the release agent provided on the surface is
so small, the release surface is amenable to being easily made
suitable for a subsequent deposition or treatment of the release
surface. Indeed, there are instances where the amount of the
release agent has no adverse effects on the subsequent treatment of
the release surface for other purposes and therefore, it is not
required to clean the release surface of residual amounts of
release agent before using the surface for other purposes. However,
in any event, the invention uses so little of the release agent on
the release surface to provide the desired non-bonding to the
surface, that little effort is required to prepare the surface for
a subsequent treatment. For this reason, the invention is
particularly desirable for use with coating applications where it
is desired to use the provision of a release surface on the object
being coated. As a result, one may arbitrarily coat the surface or
surfaces of the object, even by dipping, and the relatively thick
coating generated by dipping can be removed from the release
surface by simply brushing the surface. As a result, it is not
necessary to impart a template or mask onto the surface to prevent
coating of a select surface on the object and therefore a
complicated step is avoidable.
The preferred use of the invention is in the molding of
thermosetting resins and thermoplastics to form shaped articles in
which the interior molding surface(s) of the mold in which the
thermosetting resins and thermoplastics are to be molded are
sprayed with a release agent obtained from a solution, suspension
or dispersion of a release agent and a supercritical fluid that
vaporizes from the release agent, the thermosetting resin or
thermoplastic are supplied to the mold in contact with the interior
molding surface(s) and molded therein to effect the shape conferred
by the mold, and then the molded shaped article is removed from the
mold essentially, if not entirely, free of the release agent.
Illustrative of such thermosetting resins are, e.g., crosslinkable
acrylics, phenol-formaldehydes, alkyds, melamine-formaldehydes,
unsaturated polyesters, epoxides, and the like. Illustrative
thermoplastics include, for example, polyethylene, polypropylene,
polystyrene, polyacrylates, PVC, polycarbonates, polysulfone,
ionomers and reinforced materials.
This invention embraces as well, processes and apparatus for the
application of mold release formulations to mold surfaces wherein
the use of organic solvents is either minimized or totally
eliminated. The process comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one release agent capable of forming a thin layer or
coating on the mold surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount, such as a minor amount inclusive
of a small amount, of an active solvent or solvents capable of
dissolving, suspending or dispersing the release agent;
b. spraying said fluid mixture onto a mold surface to form a thin
layer of the release agent thereon;
c. introducing a molding composition to the mold surfaces
containing the thin layer of release agent thereon and molding the
compositions; and
d. removing the molded composition from the mold.
The invention as it applies to molding, finds exceptional utility
in a variety of molding procedures, such as reaction injection
molding (RIM), injection molding, compression molding, bulk
molding, transfer molding, cast molding, spin cast molding,
casting, vacuum forming, blow molding, calendar molding,
lamination, molding of foam, rotational molding, and the like.
This invention does not require, in most cases, that the release
agent be a material uncommon for that purpose. The process of the
invention is amenable to the employment of standard release agents
and by virtue of the manner in which the release agent is diluted
by the supercritical fluid, it is rendered useful for the practice
of the invention. Consequently, the release agent may be a liquid
or waxy material possessing the requisite release properties for
the materials to which it is applied. In a preferred embodiment,
the process comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one wax compound capable of forming a layer on the mold
surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount, such as a minor amount inclusive
of a small amount, of an active solvent or solvent(s) capable of
dissolving, suspending or dispersing the wax compound(s);
b. spraying said fluid mixture onto a mold surface to form a thin
wax layer thereon;
c. introducing a molding composition to the mold surface containing
the thin wax layer of release agent thereon and molding the
compositions; and
d. removing a molded article from contact with the mold
surface.
The invention finds its ultimate expression in the molding of
polyurethanes such as polyurethane resins and foams involving the
formation of the polyurethane per se or the polyurethane foam in an
open or closed mold. The difference between molding polyurethanes
or polyurethane foams according to the invention and according to
the prior art, is that the mold in this invention is pretreated by
spray coating predetermined areas of a solid mold surface with a
release agent obtained from a solution, suspension or dispersion of
a release agent and a supercritical fluid that vaporizes from the
release agent.
The invention includes processes and apparatus for preparation of a
mold in which the polymerization of an active hydrogen compound and
an isocyanate compound is carried out to form a molded article
conforming to the mold, wherein the mold is prepared prior to said
polymerization by the process which comprises:
a. forming a fluid mixture in a closed system, said fluid mixture
comprising:
i. at least one wax compound capable of forming a layer on the mold
surface,
ii. at least one supercritical fluid, and
iii. optionally, a reduced amount of an active solvent or
solvent(s) capable of dissolving, suspending or dispersing the wax
compound(s);
b. spraying said fluid mixture onto a mold surface to form a wax
layer thereon;
c. effecting polymerization of an active hydrogen compound and an
isocyanate compound in contact with the sprayed mold surface to
form a molded article on the surface; and
d. removing the molded article from contact with the sprayed mold
surface.
The invention is also directed to an apparatus in which the mixture
of the components of the fluid spray mixture can be blended and
sprayed onto an appropriate surface.
Thus, the invention includes an apparatus for the deposition of a
supercritical fluid blend containing a release agent which
comprises
a. closed container means for forming a supercritical fluid,
b. means for reducing the viscosity of a release agent,
c. means for combining the release agent and the supercritical
fluid and maintaining the supercritical fluid in the supercritical
fluid state, and
d. means for spraying the combination in the supercritical state to
a release surface, and
e. a release surface onto which the release agent is deposited.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a spray apparatus that can be used
in the present invention.
FIG. 2 is a schematic diagram of an another spray apparatus that
can be used in the present invention.
FIG. 3 is a perspective view of a circuit board to which is sprayed
a fine pattern of a release agent formulation depicting an
electronic circuit on the board.
FIG. 4 is a segment cross sectional side view of a dipping
operation utilizing the process of the invention.
FIG. 5 is a perspective view of wiping procedure for removing
coating at the release surfaces of a circuit board which had
undergone the treatment characterized in FIGS. 3 and 4, supra.
FIG. 6 is a perspective view shows the spraying of release agent in
accordance with the process of the invention in a baking pan prior
to addition of the food preparation.
DETAILS OF THE INVENTION
The invention relates to the use of supercritical fluids in the
spray application of release agents to a surface and the
application of a releasable material applied to the surface
containing the release agent, followed by the separation of the
releasable material from contact with the surface.
At the outset, it should be recognized that reference to
supercritical fluids as solvents for release agent will connote the
dissolving of the release agent by the supercritical fluid. The
invention is not limited to the dissolution of the release agent by
the supercritical fluid; the invention encompasses the dispersion
and suspension of the release agent by the supercritical fluid.
Therefore, where there is the tendency herein to lump solvency as
the sole function of the supercritical fluid, it is to be
understood that solvency is intended to mean that the release agent
is rendered into a more dilute flowable condition by virtue of the
supercritical fluid, and therefore, solvency means dissolving,
suspending or dispersing of the release agent by the supercritical
fluid so that the combined fluidity is characterizable by a lower
viscosity and a more fluid composition for the transport of the
release agent.
The supercritical fluid phenomenon is well documented, see pages
F-62-F-64 of the CRC Handbook of Chemistry and Physics, 67.sup.th
Edition, 1986-1987, published by the CRC Press, Inc., Boca Raton,
Fla. At high pressures above the critical point, the resulting
supercritical fluid, or "dense gas", will attain densities
approaching those of a liquid and will assume some of the
properties of a liquid. These properties are dependent upon the
fluid composition, temperature, and pressure.
The compressibility of supercritical fluids is great just above the
critical temperature where small changes in pressure result in
large changes in the density of the supercritical fluid. The
"liquid-like" behavior of a supercritical fluid at higher pressures
results in greatly enhanced solubilizing capabilities compared to
those of the "subcritical" compound, with higher diffusion
coefficients and an extended useful temperature range compared to
liquids. Compounds of high molecular weight can often be dissolved
in the supercritical fluid at relatively low temperatures. An
interesting phenomenon associated with supercritical fluids is the
occurrence of a "threshold pressure" for solubility of a high
molecular weight solute. As the pressure is increased, the
solubility of the solute will often increase by many orders of
magnitude with only a small pressure increase.
Near-supercritical liquids also demonstrate solubility
characteristics and other pertinent properties similar to those of
supercritical fluids. The solute may be a liquid at the
supercritical temperatures, even though it is a solid at lower
temperatures. In addition, it has been demonstrated that fluid
"modifiers" can often alter supercritical fluid properties
significantly, even in relatively low concentrations, greatly
increasing solubility for some solutes. These variations are
considered to be within the concept of a supercritical fluid as
used in the context of this invention. Therefore, as used herein,
the phrase "supercritical fluid" denotes a compound above, at, or
slightly below the critical temperature and pressure of that
compound.
Examples of compounds which are known to have utility as
supercritical fluids are given in Table 1. t,0220
Due to the low cost, low toxicity and low critical temperature of
carbon dioxide, supercritical carbon dioxide fluid is preferably
used in the practice of the present invention. For many of the same
reasons, nitrous oxide (N.sub.2 O) is a desirable supercritical
fluid in the practice of the present invention. However, use of any
of the aforementioned supercritical fluids and mixtures thereof are
to be considered within the scope of the present invention.
The solvency of supercritical carbon dioxide is similar to that of
a lower aliphatic hydrocarbon and, as a result, one can consider
supercritical carbon dioxide as a replacement for the hydrocarbon
solvent of a conventional mold release formulation. In addition to
the environmental benefit of replacing hydrocarbon solvents with
supercritical carbon dioxide, there is a safety benefit also,
because carbon dioxide is nonflammable and nontoxic.
The purpose of the invention is to utilize such compounds in
combination with mold release agents for applying the agents to a
release surface. The utility of any of the above-mentioned
compounds as supercritical fluids in the practice of the present
invention will depend upon the release agent, whether it is a wax
material or a liquid, whether there is present an active solvent,
and the like considerations.
Release agents come in many forms and compositions. Most mold
release agents are waxes, waxlike or greases. Release agents for
food products, such as hydrogenated vegetable oils such as
shortenings, lecithin, and the like, are solid waxlike or
grease-like materials at operative food preparation temperatures.
In addition, there are liquids that can be used as mold release
agents.
Considering the various attributes of a release agent, it is
desired that the release agent be a material that has minimum flow
characteristics on the release surface. It is desired that the
release agent not have such flow on the release surface that the
release agent interferes with the uniform application of the
material to be released from the release surface. Therefore, on
application, the release agent should deposit and remain
essentially fixed to the surface that it is deposited on without
any substantial migration from the point of deposition.
A significant advantage of the invention is that it provides a thin
uniform layer of the release agent on the release surface. This
layer is usually considerably thinner than that created by the
usual spraying of the release agent onto the release surface.
Though the layer of the release agent on the release surface can be
a continuous film on the release surface, it is not necessary that
it be such a film. However, when the release surface is heated,
there is greater likelihood that a continuous film will be formed.
If the release agent has flow when deposited on the release
surface, there will be a tendency for coalescence of the sprayed
particles on the release surface. Such coalesence will typically
result in the fusion of particles whereby to form a continuous film
of such fused particles. Even with such fusion, it is not necessary
that in effecting fusion the particles lose the identity of the
particulate state. Thus a continuous film of such particles can be
formed which maintains the identity of the particles by providing
the particle shapes in the film. In such a case, the topology of
the film is irregular reflecting the uniqueness of each of the
particles. However, if the sprayed particles of release agent do
not coalesce on the release surface, then the layer of release
agent can be a thin mass of discrete particles on the release
surface. The invention accommodates the use of a layer of discrete
particles of release agent in which a substantial portion of the
particles are in a noncoalesced intermittent pattern. In such a
case, even though portions of the release surface are openly
exposed to the material deposited over the release agent, the small
size of the release agent particles and the density of the layer of
the particles on the surface protect the surface from being
contacted by the material. It is this combination of particle size
and density of the layer that assures the good release qualities.
The particle size of the release agent deposited on the release
surface is not narrowly critical. These particles may be liquid or
solid. If the particles are extremely small, then it may prove
necessary to spray the surface longer to achieve the desired
density of the release agent coating on the release surface. If the
particles are very large, then it may prove desirable to effect
flow of the release agent on the release surface to cause as much
coalescence as will effect the desired coverage to achieve the
desired level of release. As a rule, the release agent particles
supplied in the spray to the surface will be at least one (1)
micron (.mu..) in diameter, preferably from about 2 to about 100
.mu.., and most preferably from about 5 to about 50 .mu.. The
thickness of the release agent coating can vary greatly and thus,
the film thickness of the release agent is not a narrowly critical
limitation in the practice of the invention. It will be most common
to want to put a coating on the release surface that is at least 1
.mu.. thick but is not thicker than 100 .mu.. In unusual cases, the
thickness of the coating will be as great as 4 mils, and the usual
thickness will be significantly less than that.
Release agents used for molding purposes are wax, waxlike, greases
or liquids that are derived from petroleum sources or contain
silicones (viz., polydimethylsiloxanes) or constitute a combination
of the two or are salts of long chain saturated fatty acids such as
stearic acid. The choice of release agent is tied to the
composition being molded or otherwise being applied to the release
surface. It is desired that the release agent not be so compatible
with the material being applied to it that the agent dissolves in
the material.
There is commercially available a wide range of chemicals being
offered as release agents. They vary from such compositions as:
ethylene bis-stearamide, water soluble sulfated oil, dioctyl ester
of sodium sulfosuccinic acid, nonpolar solvents and petroleum oils,
monoalkyl primary amines, straight chain aliphatic hydrocarbons,
polyolefins, hydrogenated castor oil, methyl hydroxystearate,
mixture of esters, fatty acids in petroleum oil base, blends of
esters, blend of fatty acid derivatives and surface active
compounds, dimethyl silicone fluids, silicone mica -glycol
emulsion, single, double, and triple pressed and food grade stearic
acid, single and double pressed oleic acids distilled, phosphated
mono and diglycerides, modified fatty acid amide, emulsion of
dimethyl siloxane, amide wax, oleyl palmitamide, stearyl erucamide,
calcium stearate, zinc stearate, potassium and sodium ricinoleate,
microcrystalline wax, N-(2-hydroxyethyl) 12-hydroxystearamide,
polyvinylpolypyrrolidone, crystalline, aliphatic, saturated
polyethylene of 500 molecular weight, crystalline, aliphatic,
saturated high density polyethylene of 700 molecular weight,
crystalline, aliphatic, saturated high density polyethylene of 1000
molecular weight, crystalline, aliphatic, saturated high density
polyethylene of 2000 molecular weight, emulsifiable high density
polyethylene, fatty amido-amine salt, fatty amide, synthetic waxes,
silicone oxyalkylene copolymers, methyl phenyl silicones, lecithin,
surfactants such as nonionics, anionics, cationics and amphoterics,
and the like.
The wax compounds suitable for use in this invention as mold
release waxes are any of the waxes known to those skilled in the
art of mold release formulations. In general, wax refers to a
substance which is a plastic solid at ambient temperature but at
moderately elevated temperatures becomes a low viscosity liquid.
These include insect and animal waxes as well as petroleum waxes,
polyethylene waxes, Fischer-Tropsch waxes, chemically modified
hydrocarbon waxes and substituted amide waxes.
The silicone release agents are typically based on silicone liquid
compositions such as polydimethylsiloxanes or
polymethylphenylsiloxanes having a methyl or methyl and phenyl to
silicon ratio of at least about 2, preferably 2 or greater than 2.
They can be blended with hydrocarbon materials, such as solvents
and waxes.
Illustratively, the release agent such as a wax component of a mold
release composition, is generally present in amounts ranging from
0.1 to 30 wt % based upon the total weight of the mold release
composition. Preferably, the wax component would be present in
amounts ranging from 0.5 to 20 wt % on the same basis.
As pointed out above, the release agent may be employed in the
practice of the invention without the use of a solvent other than
the supercritical fluid solvent. The active solvent(s) other than
the supercritical fluid suitable in the practice of this invention
includes any solvent or mixture of solvents which is capable of
dissolving, dispersible or suspending the release agent system in
combination with the supercritical fluid. It is quite apparent that
the selection of solvent will be dependent upon the release agent
that is used. Since most release agents are oleophilic, the
solvents will typically be hydrocarbon based materials.
Generally, solvents suitable for this invention must have the
desired solvency characteristics as aforementioned and also the
proper balance of evaporation rates so as to insure good coating
formation of the release agent. A review of the structural
relationships important to the choice of solvent or solvent blend
is given by Dileep et al, Industrial and Engineering Chemistry
Product Research and Development 24, 162, 1985 and Francis, A. W.,
Journal of Physical Chemistry 58, 1099, 1954.
In order to diminish or minimize the unnecessary volatilization of
any active solvent present in the fluid spray mixture, the amount
of active solvent used should be less than that required to produce
a mixture of release agent and active solvent having a viscosity
which will permit its application by fluid spray techniques. In
other words, the inclusion of active solvent(s) should be
diminished or minimized such that the diluent effect due to the
presence of the supercritical fluid diluent is fully utilized.
Suitable active solvents include: aliphatic hydrocarbons such as
hexane, heptane, octane, nonane, decane, undecane, dodecane, and
other higher molecular weight aliphatic hydrocarbons; aromatic
hydrocarbons such as benzene, toluene, xylene and other aromatics,
either singly or in mixtures; halogenated aliphatic and aromatic
hydrocarbons such as halogenated methanes, ethanes, propanes, and
higher molecular weight homologs, as well as halogenated benzenes,
and the like; oxygenated solvents such as alcohols, ketones,
aldehydes, ethers, esters, glycol ethers, glycol ether esters and
others; water; surface active compounds such as nonionic, anionic,
cationic and amphoteric surfactants.
In general, the amount of active solvent(s) (other than the
supercritical fluid) should be minimized so that the beneficial
effect due to the presence of the supercritical fluid is maximized.
It is preferred that the only solvent used with the release agent
is the supercritical fluid. However, the desired solvency,
dispersibility or suspensionability of the release agent may not be
achieved using the supercritical fluid alone. In that case, the
other active solvents are provided in the release agent
formulation. Overall, the other solvent(s) should be present in
amounts ranging from 0 to about 70 weight percent based upon the
total weight of the release agent(s), solvent(s), and supercritical
fluid, which in this case is termed a diluent. In such a case, the
solvent(s) is more typically present in the formulation of the
release agent formulation in the range of from 0.15 wt % to 60 wt %
bases upon the weight of the total mold release composition, and
most preferably, between 0.3 wt % and 30 wt % on the same basis.
Most preferably, the solvent(s) are present in amounts ranging from
about 0.5 to 30 weight percent on the same basis. The choice of wax
compound(s) and active solvent(s) other than the supercritical
fluid solvent should take into consideration the fact that the
spray temperature cannot exceed the temperature at which thermal
degradation of any component of the fluid spray mixture occurs.
Therefore, these components should not degrade under the spray
conditions.
The supercritical fluid diluent should be present in such amounts
that a fluid mixture is formed that possesses such a viscosity that
it may be applied as a fluid spray.
If supercritical carbon dioxide fluid is employed as the
supercritical fluid diluent, i.e., another active solvent is
present, preferably CO.sub.2 should be present in the mixture with
the release agent and the other active solvent(s) in amounts
ranging from about 10 to about 95 weight percent based upon the
total weight of components forming the sprayable release agent
formulation. Most preferably, it is present in amounts ranging from
about 20 to about 95 weight percent on the same basis.
If a release agent is mixed with increasing amounts of
supercritical fluid in the absence of another active solvent, the
composition may at some point separate into two distinct phases.
Prior to this condition, the addition of the supercritical fluid
such as supercritical carbon dioxide fluid will have reduced the
viscosity of the viscous release agent composition to a range where
it can be readily atomized such as by passing it through a spray
orifice of an airless spray gun. After atomization, a majority of
the carbon dioxide vaporizes, leaving substantially the composition
of the original release agent formulation. Upon contacting the
substrate, the remaining fluid mixture of release agent and
solvent(s) component(s) will flow to produce a thin, uniform,
smooth film on the substrate. If the release agent is a wax, and
another active solvent is not used, then the release agent may be
solidified as fine particles that are uniformly deposited onto the
release surface.
It is to be understood that a specific sequence of addition of the
components of the mold release composition is not necessary in the
practice of the present invention. However, it is often preferred
to initially mix the release agent, such as a wax release agent,
and the active solvent(s) other than the supercritical fluid, if
they are employed.
The process and apparatus of the invention comprise means for
effecting a pressurized mixture containing the release agent and
the supercritical fluid, means for spraying the pressurized mixture
to a release surface, a release surface onto which the release
agent is deposited, means for the introduction of a material that
is to be released from the release surface, and the step of
releasing the material from contact with the release agent and the
release surface.
In that context, the pressurized mixture of the release agent
dissolved, suspended or dispersed in the supercritical fluid is
transported to the nozzle of the spray device where the fluid
containing the release agent is rapidly issued through a relatively
narrow orifice into an expanded area which causes an immediate
pressure drop. This rapid release of pressure tends to cause the
supercritical fluid to expand to a gas or vapor immediately, at an
expansion rate far greater than the more dense release agent and
any active solvent that accompanies the release agent. The release
agent and any accompanying solvent is broken into discrete
particles and the gaseous or vaporous component which was the
supercritical fluid disappears from the particles into the general
atmosphere.
The spray pressure used in the practice of the present invention is
a function of the release agent formulation, the supercritical
fluid being used, and the viscosity of the liquid mixture. The
minimum spray pressure is at or slightly below the critical
pressure of the supercritical fluid. Generally the pressure will be
below about 5000 psi. Preferably the spray pressure is above the
critical pressure of the supercritical fluid and below about 3000
psi. If the supercritical fluid is supercritical carbon dioxide
fluid, the preferred spray pressure is between about 1070 psi and
about 3000 psi. The most preferred spray pressure is between about
1200 psi and about 2500 psi.
The spray temperature used in the practice of the present invention
is a function of the release agent formulation, the supercritical
fluid being used, and the concentration of supercritical fluid in
the liquid mixture. The minimum spray temperature is at or slightly
below the critical temperature of the supercritical fluid. The
maximum temperature is the highest temperature at which the
components of the liquid mixture are not significantly thermally
degraded during the time that the liquid mixture is at that
temperature.
If the supercritical fluid is supercritical carbon dioxide fluid,
because the supercritical fluid escaping from the spray nozzle
could cool to the point of condensing solid carbon dioxide and any
ambient water vapor present due to high humidity in the surrounding
spray environment, the spray composition is preferably heated prior
to atomization. The minimum spray temperature is about 31.degree.
C. The maximum temperature is determined by the thermal stability
of the components in the liquid mixture. The preferred spray
temperature is between 35.degree. C. and 90.degree. C. The most
preferred temperature is between 45.degree. C. and 75.degree. C.
Generally liquid mixtures with greater amounts of supercritical
carbon dioxide fluid require higher spray temperatures to
counteract the greater cooling effect.
Typically the spray undergoes rapid cooling while it is close to
the orifice, so the temperature drops rapidly to near or below
ambient temperature. If the spray cools below ambient temperature,
entrainment of ambient air into the spray warms the spray to
ambient or near ambient temperature before the spray reaches the
substrate. This rapid cooling is beneficial, because less active
solvent(s) evaporates in the spray in comparison to the amount of
solvent lost in conventional heated airless sprays. Therefore a
greater proportion of the active solvent is retained in the release
agent formulation to aid leveling of the release agent on the
release surface substrate. Conventional heated airless sprays also
cool to ambient temperature before reaching the release surface
substrate, because of solvent evaporation and entrainment of
ambient air.
The spray temperature may be obtained by heating the liquid mixture
before it enters the spray gun, by heating the spray gun itself, by
circulating the heated liquid mixture to or through the spray gun
to maintain the spray temperature, or by a combination of methods.
Circulating the heated liquid mixture through the spray gun is
preferred, to avoid heat loss and to maintain the desired spray
temperature. Tubing, piping, hoses, and the spray gun are
preferably insulated or heat traced to prevent heat loss.
The environment in which the liquid spray of the present invention
is conducted is not narrowly critical. However, the pressure
therein must be less than that required to maintain the
supercritical fluid component of the liquid spray mixture in the
supercritical state. Preferably, the present invention is conducted
in air under conditions at or near atmospheric pressure. Other gas
environments can also be used, such as air with reduced oxygen
content or inert gases such as nitrogen, carbon dioxide, helium,
argon, xenon, or a mixture. Oxygen or oxygen enriched air is not
desirable, because oxygen enhances the flammability of organic
components in the spray.
The present process may be used to apply release agents by the
application of liquid spray to a variety of release surface
substrates. The choice of substrates is therefore not critical in
the practice of the present invention. Examples of suitable
substrates include but are not limited to metal, wood, glass,
plastic, paper, cloth, ceramic, masonry, stone, cement, asphalt,
rubber, and composite materials. The substrate may be a conductor
or a dielectric.
There are a broad variety of spray devices that one may use in
carrying out the invention. Essentially any spray gun may be used,
from conventional airless and air-assisted airless spray devices to
electrostatic spray devices. The choice of spray device is
dependent upon the kind of application in which the invention is
used.
Airless spray uses a high pressure drop across the orifice to
propel the release agent formulation through the orifice at high
velocity. Upon exiting the orifice, the high-velocity liquid breaks
up into droplets and disperses into the air to form a liquid spray.
Sufficient momentum remains after atomization to carry the droplets
to the substrate. The spray tip is contoured to modify the shape of
the liquid spray, which is usually a round or elliptical cone or a
flat fan. Turbulence promoters are sometimes inserted into the
spray nozzle to aid atomization. Spray pressures typically range
from 700 to 5000 psi. The pressure required increases with fluid
viscosity.
Air-assisted airless spray combines features of air spray and
airless spray. It uses both compressed air and high pressure drop
across the orifice to atomize the release agent formulation and to
shape the liquid spray, typically under milder conditions than each
type of atomization is generated by itself. Generally the
compressed air pressure and the air flow rate are lower than for
air spray. Generally the liquid pressure drop is lower than for
airless spray, but higher than for air spray. Liquid spray
pressures typically range from 200 to 800 psi. The pressure
required increases with fluid viscosity.
The present invention may utilize compressed gas to assist
formation of the liquid spray and/or to modify the shape of the
liquid spray that comes from the orifice. The assist gas is
typically compressed air at pressures from 5 to 80 psi, with low
pressures of 5 to 20 psi preferred, but may also be air with
reduced oxygen content or inert gases such as compressed nitrogen,
carbon dioxide, helium, argon, or xenon, or a mixture. Compressed
oxygen or oxygen enriched air is not desirable, because oxygen
enhances the flammability of the organic components in the spray.
The assist gas is directed into the liquid spray as one or more
high-velocity jets of gas, preferably arranged symmetrically on
each side of the liquid spray to balance each other. The assist gas
jets will preferably come from gas orifices built into the spray
tip and/or nozzle. The assist gas may also issue from an opening in
the spray tip or nozzle that is a concentric annular ring that is
around and centered on the liquid orifice, to produce a hollow-cone
high-velocity jet of gas that converges on the liquid spray, but
this creates a larger flow of assist gas that is not as desirable.
The concentric annular ring may be divided into segments, to reduce
gas flow rate, and it may be elliptical instead of circular, to
shape the spray. Preferably the flow rate and pressure of the
assist gas are lower than those used in air spray. The assist gas
may be heated to counteract the cooling effect of the supercritical
fluid diluent in the liquid spray.
Airless spray and air-assisted airless spray can also be used with
the liquid release agent formulation heated or with the air heated
or with both heated. Heating reduces the viscosity of the liquid
release agent formulation and aids atomization.
The fluid mixture of the release agent and the supercritical fluid
is sprayed onto a substrate to form a coating thereon by passing
the fluid mixture under pressure through an orifice into the
environment of the substrate to form a fluid spray. An orifice is a
hole or an opening in a wall or housing, such as in a spray tip of
a spray nozzle on an airless spray gun, through which the fluid
mixture of the release agent with or without active solvent and the
supercritical fluid flows in going from a region of higher
pressure, such as inside the spray gun, into a region of lower
pressure, such as the air environment outside of the spray gun and
around the substrate. An orifice may also be a hole or an opening
in the wall of a pressurized vessel, such as a tank or cylinder. An
orifice may also be the open end of a tube or pipe or conduit
through which the mixture is discharged. The open end of the tube
or pipe or conduit may be constricted or partially blocked to
reduce the open area.
Spray orifices, spray tips, spray nozzles, and spray guns used for
conventional electrostatic, airless and air-assisted airless
spraying of coating formulations such as paints, lacquers, enamels,
and varnishes, are suitable for spraying release agent formulations
with supercritical fluids, that is, for spraying the supercritical
fluid containing mixture of the invention. Spray guns, nozzles, and
tips are preferred that do not have excessive flow volume between
the orifice and the valve that turns the spray on and off. The
spray guns may be automatic or hand spray. The spray guns, nozzles,
and tips must be built to contain the spray pressure used.
The material of construction of the orifice is not critical in the
practice of the present invention, provided the material possesses
necessary mechanical strength for the high spray pressure used, has
sufficient abrasion resistance to resist wear from fluid flow, and
is inert to chemicals with which it comes into contact. Any of the
materials used in the construction of airless spray tips, such as
boron carbide, titanium carbide, ceramic, stainless steel or brass,
is suitable, with tungsten carbide generally being preferred.
The orifice sizes suitable for the practice of the present
invention generally range from about 0.004-inch to about 0.072-inch
diameter. Because the orifices are generally not circular, the
diameters referred to are equivalent to a circular diameter. The
proper selection is determined by the orifice size that will supply
the desired amount of release agent and accomplish proper
atomization for the release agent. Generally smaller orifices are
desired at lower viscosity and larger orifices are desired at
higher viscosity. Smaller orifices give finer atomization but lower
output. Larger orifices give higher output but poorer atomization.
Finer atomization is preferred in the practice of the present
invention. Therefore small orifice sizes from about 0.004-inch to
about 0.025-inch diameter are preferred. Orifice sizes from about
0.007-inch to about 0.015-inch diameter are most preferred.
The designs of the spray tip that contains the spray orifice and of
the spray nozzle that contains the spray tip are not critical to
the practice of the present invention. The spray tips and spray
nozzles should be essentially free of protuberances near the
orifice that could/would interfere with the spray.
The shape of the spray is not critical to the practice of the
present invention but it can be important in some applications of
the invention. The spray may be in the shape of a cone that is
circular or elliptical in cross section or the spray may be in the
shape of a flat fan, but the spray is not limited to these shapes.
Sprays that are flat fans or cones that are elliptical in cross
section are preferred for applications requiring a broad sweeping
deposition of the release agent. In those cases, wide-angle fans
are most preferred.
The distance from the orifice to the release surface is not
critical to the practice of the present invention. Generally the
substrate in which a broad deposition of the release agent is
effected will be sprayed from a distance of about 4 inches to about
24 inches. A distance of 6 inches to 18 inches is preferred. A
distance of 8 inches to 14 inches is most preferred.
Devices and flow designs that promote turbulent or agitated flow in
the liquid mixture prior to passing the liquid mixture under
pressure through the orifice may also be used in the practice of
the present invention. Such techniques include but are not limited
to the use of pre-orifices, diffusers, turbulence plates,
restrictors, flow splitters/combiners, flow impingers, screens,
baffles, vanes, and other inserts, devices, and flow networks that
are used in electrostatic, airless spray and air-assisted airless
spray.
Filtering the liquid mixture prior to flow through the orifice is
desirable in the practice of the present invention in order to
remove particulates that might plug the orifice. This can be done
using conventional high-pressure paint filters. A filter may also
be inserted at or in the gun and a tip screen may be inserted at
the spray tip to prevent orifice plugging. The size of the flow
passages in the filter should be smaller than the size of the
orifice, preferably significantly smaller.
Electrostatic forces are commonly utilized with orifice sprays such
as air spray, airless spray, and air-assisted airless spray to
increase the proportion of fluid release agent that is deposited
onto the substrate from the fluid spray. This is commonly referred
to as increasing the transfer efficiency. This is done by using a
high electrical voltage relative to the substrate to impart a
negative electrical charge to the spray. The substrate is
electrically grounded. This creates an electrical force of
attraction between the fluid spray particles and the release
surface, which causes particles that would otherwise miss the
surface to be deposited onto it. When the electrical force causes
particles to be deposited on the edges and backside of the
substrate, this effect is commonly referred to as wrap around. The
release surface should be electrically conducting or be given a
conducting surface before being sprayed.
The fluid spray can be electrically charged at any stage of the
spray formation process. It can be charged by applying high
electrical voltage and electrical current (1) within the spray gun,
by direct contact with electrified walls or internal electrodes
before passing through the orifice; (2) as the fluid emerges from
the orifice, by electrical discharge from external electrodes
located near the orifice and close to the spray; or (3) away from
the orifice, by passing the fluid spray through or between
electrified grids or arrays of external electrodes before the spray
reaches the release surface.
Electrically charging the fluid spray as it emerges from the
orifice is widely used. Usually a short pointed metal wire, which
extends from the spray nozzle to beside the spray, is used as the
electrode. When a high electrical voltage is applied to the
electrode, electrical current flows from the point of the electrode
to the fluid spray, which becomes charged. This method is used for
air spray, airless spray, and air-assisted airless spray guns. It
is used for both hand spray guns and automatic spray guns.
Generally the electrical voltage ranges from 30 to 150 kilovolts.
Release agent formulations that are sufficiently conductive will
leak electrical charge through the fluid to the material supply
system; these systems must be isolated from electrical ground so
that the system itself becomes electrified. For safety reasons, the
voltage of hand spray guns is usually restricted to less than 70
kilovolts and the equipment is designed to automatically shut off
the voltage when the current exceeds a safe level. Generally for
hand spray guns the useful range of electrical current is between
20 and 100 microamperes and optimum results are obtained with
release agent formulations that have very low electrical
conductivity, that is, very high electrical resistance.
The invention is specifically directed to a fluid spray process in
which the fluid spray mixture of the release agent and the
supercritical fluid is electrically charged by a high electrical
voltage relative to the substrate. Preferably the substrate is
grounded, but it may also be charged to the opposite sign as the
fluid mixture or spray. The substrate may be charged to the same
sign as the fluid mixture or spray, but at a lower voltage with
respect to ground, but this is of less benefit, because this
produces a weaker electrical force of attraction between the spray
and the substrate than if the substrate were electrically grounded
or charged to the opposite sign. Electrically grounding the
substrate is the safest mode of operation. Preferably the fluid
mixture and/or fluid spray is charged negative relative to
electrical ground.
The method of electrostatically charging the release
agentsupercritical fluid mixture and/or spray is not critical to
the practice of the invention provided the charging method is
effective. The fluid mixture can be electrically charged by
applying high electrical voltage relative to the substrate and
electrical current (1) within the spray gun, by direct contact with
electrified walls or internal electrodes before passing through the
orifice; (2) as the fluid emerges from the orifice, by electrical
discharge from external electrodes located near the orifice and
close to the spray; or (3) away from the orifice, by passing the
fluid spray through or between electrified grids or arrays of
external electrodes before the spray is deposited onto the
substrate. Methods (1) and (2), individually or in combination, are
preferred. Method (2) is most preferred. In charging method (1)
above, the spray gun must be electrically insulating. The high
voltage and electrical current is supplied to the fluid mixture
inside the gun by direct contact with an internal surface that is
electrically conducting and electrified. This may be part of the
wall of the flow conduit inside the gun or internal electrodes that
extend into the flow or a combination of electrified elements,
including the spray nozzle. The contact area must be large enough
to transfer sufficient electrical charge to the fluid mixture as it
flows through the gun. This internal charging method has the
advantage of having no external electrode that could interfere with
the spray. A disadvantage is that if the fluid mixture is not
sufficiently electrically insulating, electrical current leakage
can occur through the fluid mixture to a grounded feed supply tank
or feed delivery system. This reduces the amount of charge going to
the spray. If current leakage is too high, then the feed supply
tank and feed delivery system must be insulated from electrical
ground, that is, be charged to high voltage. Current leakage can be
measured by measuring the current flow from the high voltage
electrical power supply without fluid flow. The current charging
the spray is then the difference between the current with fluid
flow and the current without fluid flow. The leakage current should
be small compared to the charging current.
In charging method (2) above, the fluid spray is electrically
charged as it emerges from the orifice or in the vicinity of the
orifice. The spray gun and spray nozzle must be electrically
insulating. The electrical charge is supplied from external
electrode(s) close to the spray tip and adjacent to the spray.
Under high electrical voltage, electrical current is discharged to
the spray. The preferred electrodes are one or more metal wire(s)
positioned adjacent to the spray. The electrodes may be either
parallel to the spray or perpendicular to it or any orientation
inbetween such that the electrical current issuing from the point
is favorably directed to the fluid spray. The electrode(s) must be
positioned close enough to the spray, preferably within one
centimeter, to effectively charge the spray without interfering
with the flow of the spray. The electrodes may be sharp pointed and
may be branched. For planar sprays, one or more electrodes are
preferably located to the side(s) of the planar spray so that
electrical current is discharged to the face(s) of the spray. For
oval sprays, one or more electrodes are located adjacent to the
spray around the perimeter. The electrode(s) are located to
effectively charge the spray. One or more auxiliary electrodes,
which may be at a different voltage than the primary electrode(s)
or electrically grounded, may be used to modify the electrical
field or current between the primary electrode(s) and the spray.
For example, a primary charging electrode may be on one side of the
spray fan and a grounded insulated auxiliary electrode may by on
the opposite side of the spray fan. Charging method (2) has the
advantage of less current leakage through the fluid mixture than
charging method (1). Fluid mixtures that are sufficiently
conductive must have the feed supply and feed line insulated from
electrical ground.
In charging method (3) above, the fluid spray is electrically
charged farther away from the orifice and is more fully dispersed
than in method (2). Therefore a larger network of external
electrodes is required in order to effectively charge the spray.
Therefore the method is less safe and less versatile. Also the
distance between the electrodes and spray must be greater to avoid
interfering with the spray. Therefore the charge applied to the
spray is likely to be lower. But current leakage through the supply
lines is eliminated. The fluid spray is passed through or between
electrified grids or arrays of external electrodes before the spray
is deposited onto the substrate. The spray particles are charged by
ion bombardment from the electrical current discharged into air
from the electrodes.
The present invention can be used with high electrical voltage in
the range of about 30 to about 150 kilovolts. Higher electrical
voltages are favored to impart higher electrical charge to the
fluid spray to enhance attraction to the substrate, but the voltage
level must be safe for the type of charging and spray gun used. For
safety reasons, the voltage of hand spray guns is usually
restricted to less than 70 kilovolts and the equipment is designed
to automatically shut off the voltage when the current exceeds a
safe level. Generally for hand spray guns the useful range of
electrical current is between 20 and 200 microamperes and optimum
results are obtained with coating formulations that have very low
electrical conductivity, that is, very high electrical resistance.
For automatic spray guns that are used remotely, higher voltages
and electrical currents can be safely used than for hand spray
guns. Therefore the voltage can exceed 70 kilovolts up to 150
kilovolts and the current can exceed 200 microamperes.
These methods of electrostatic charging are known to those who are
skilled in the art of conventional electrostatic spraying.
Supercritical carbon dioxide fluid surprisingly has been found to
be an insulating solvent with good electrical properties for
electrostatic spraying. The fluid sprays give good electrostatic
wrap around the substrate. This shows that the particles are highly
charged and retain the electric charge.
Humid air can cause electrostatic sprays to lose their electrical
charge more quickly than dry air; hence the electrostatic
attraction to the substrate and wrap around is less effective. The
supercritical carbon dioxide fluid diluent is beneficial for
spraying in a humid environment, because the carbon dioxide as it
vents from the spray tends to displace the humid air surrounding
the spray. This helps the spray to retain its electric charge
longer. When compressed air is used to assist electrostatic
atomization, dry air is favored over humid air.
For electrostatic spraying, the substrate is preferably an
electrical conductor such as metal. But substrates that are not
conductors or semiconductors can also be sprayed. Preferably they
are pretreated to create an electrically conducting surface. For
instance, the substrate can be immersed in a special solution to
impart conductivity to the surface.
The method of generating the high electrical voltage and electrical
current is not critical to the practice of the current invention.
High voltage electrical power supplies can be used in the same way
as in conventional electrostatic spraying. The power supply should
have standard safety features that prevent current or voltage
surges. The electrical power supply may be built into the spray
gun. Other charging methods may also be used.
More information about orifice sprays such as air spray, airless
spray, and air-assisted airless spray, about heated orifice sprays,
and about electrostatic spraying can be obtained from the general
literature of the coating industry and from technical bulletins
issued by spray equipment manufacturers, such as the following
references:
a. Martens, C. R., Editor. 1974. Technology of Paints, Varnishes
and Lacquers. Chapter 36. Application. Robert E. Krieger Publishing
Company, Huntington, N.Y.
b. Fair, James. 1983. Sprays. Pages 466-483 in Grayson, M., Editor.
Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition.
Volume 21. Wiley-Interscience, N.Y.
c. Zinc, S. C. 1979. Coating Processes. Pages 386-426 in Grayson,
M., Editor. Kirk-Othmer Encyclopedia of Chemical Technology. Third
Edition. Volume 6. Wiley-Interscience, N.Y.
d. Long, G. E. 1978 (Mar. 13). Spraying Theory and Practice.
Chemical Engineering: 73-77.
e. Technical Bulletin. Air Spray Manual. TD10-2R. Binks
Manufacturing Company, Franklin Park, Ill.
f. Technical Bulletin. Compressed Air Spray Gun Principles.
TD10-1R-4. Binks Manufacturing Company, Franklin Park, Ill.
g. Technical Bulletin. Airless Spray Manual. TD11-2R. Binks
Manufacturing Company, Franklin Park, Ill.
h. Technical Bulletin. Airless Spraying. TD11-1R-2. Binks
Manufacturing Company, Franklin Park, Ill.
i. Technical Bulletin. Electrostatic Spraying. TD17-1R. Binks
Manufacturing Company, Franklin Park, Ill.
j. Technical Bulletin. Hot Spraying. TD42-1R-2. Binks Manufacturing
Company, Franklin Park, Ill.
k. Technical bulletin on air-assisted airless spray painting
system. Kremlin, Incorporated, Addison, Ill.
U.S. Pat. Nos. 3,556,411; 3,647,147; 3,754,710; 4,097,000; and
4,346,849 disclose spray nozzles and tips for use in airless spray,
including designs and methods of manufacture and methods of
promoting turbulence in the atomizing fluid. U.S. Pat. No.
3,659,787 discloses a spray nozzle and use of electrostatics for
airless spray. U.S. Pat. Nos. 3,907,202 and 4,055,300 disclose
spray nozzles and use of electrostatics for air-assisted airless
spray. None of these patents uses supercritical fluids as diluents
to spray release agent formulations.
With respect to FIG. 1, there is shown a schematic diagram of
supercritical carbon dioxide batch unit for spraying release agent
formulations. Liquid carbon dioxide (bone-dry grade) cylinder (or
any other source of CO.sub.2) 1 provided with with eductor tube
outlet 3, valve 5 and pressure gauge 7, supplies CO.sub.2 to the
feed tank 31 via valves 9, 17, 23 and 25, and lines 19, 21, 27 and
29. Valve 28 is closed to allow CO.sub.2 feed to tank 31. Feed tank
31 is supported by frame 33 on weight scale 35 for monitoring the
amount of CO.sub.2 in tank 31. Tank 31 is provided with exit pipe
39 and the pressure in tank 31 is controlled by pressure gauge 41,
pressure relief valve 43 and valve 37. When the CO.sub.2 pressure
in tank 31 reaches the desired value, feed from cylinder 1 is cut
off by closing valve 25 or valve 17, or both. CO.sub.2 is fed to
the system via line 29 by opening valve 28, through line 45, valve
47 and pump 49. Pump 49 may be a Haskel.RTM. air drive piston pump
(Haskel Incorporated, Engineered Products Division, 100 E. Graham
Place, Burbank, Calif. 91502). The purpose of pump 49 is to
maintain the desired feed rate into vessel 59 through valves 53 and
57 and lines 51 and 55. Vessel 59 is a high pressure, agitated
jacketed tank for blending the CO.sub.2 with the release agent. In
this particular embodiment, vessel 59 has a 10 liter capacity.
Vessel 59 is provided with agitator means 61, thermocouples 62 and
64, and charging funnel 71 fitted to valve 69. At the bottom of
vessel 59 are discharge line 63 fitted with a valve and heat
controlled spray connection line 67 connected to vessel 59 via
valve 65. Line 67 may be electrically heated to maintain the
necessary supercritical temperature of the supercritical carbon
dioxide fluid. Vessel 59 is fitted with vent line 77 connected to
valve 79 connected to discharge vent line 81. The pressure in
vessel 59 is monitored by pressure gauge 75 and controlled by
pressure relief valve 73.
The supercritical fluid - release agent mixture is fed via heated
line 67 fitted with thermocouple 83, and a thermocouple (not shown)
at the start of line 67, to spray gun 85. The choice of spray gun
is not narrowly critical. A wide variety of spray guns are
available ranging from an artists spray gun that will draw fine
lines to spray guns that generate a wide spray for the typical
industrial applications encompassed by the invention. The spray 87
of the release agent is directed toward the release surface 89.
Carbon dioxide is separated from the spray as the mixture is
discharged from the spray nozzle and the emitted CO.sub.2 is either
vented to a CO.sub.2 recovery system or to the atmosphere.
With respect to FIG. 2, there is schematically illustrated a
continuous process and an apparatus assembly for spraying a release
agent formulation onto a release surface. Liquid carbon dioxide
(bone-dry grade) cylinder 90 supplies CO.sub.2 to pump 92 via line
91. The CO.sub.2 is typically fed at about ambient temperature. In
pump 92, the CO.sub.2 is brought to a supercritical pressure such
as about 1200 psi. The pressurized CO.sub.2 is passed to heat
exchanger 94 through line 93. Supercritical CO.sub.2 formed in heat
exchanger 94 is passed through line 95 and pressure relief valve 96
to mixing chamber 97, containing an impingement manifold utilizing
a static mixer (not shown). The release agent is prepared for
mixing with the supercritical CO.sub.2 in chamber 97 by adding the
release agent with the solvent, if employed, via funnel 101 to a
mixing tank 98 provided with stirrer 99. If the release agent is a
wax, then mixing tank 98 is heat jacketed to bring the wax solvent
mixture to a fluid condition. The fluid release agent mixture is
removed from tank 98 through heat traced line 103 using pump 105.
Line 107 may be electrically heated to maintain the necessary
viscosity of the wax solvent mixture. From pump 105, the fluid
mixture is passed by way of heat traced line 107 to valve 109.
Recycle of the fluid mixture is effected through line 111 for
effective temperature control. A part of the fluid mixture is
passed through line 113 to mixing chamber 97 where it is dissolved,
suspended or dispersed in the supercritical carbon dioxide fluid.
The supercritical fluid mixture containing the release agent is
carried from chamber 97 via line 115, through open valve 117 to
line 119, and to high pressure vessel 121 which is fitted with vent
line 135 connected to valve 139. The pressure in vessel 121 is
controlled by pressure relief valve 141.
The supercritical mixture of the release agent-solvent mixture and
the supercritical CO.sub.2 is fed to holding vessel 121 containing
stirrer 125 and the mixture of the supercritical fluid - release
agent mixture is fed via heated line 129, through valve 127, to
spray gun 131 and sprayed 133 onto the release surface.
With respect to FIG. 3, there is shown a perspective view of a
circuit board to which is sprayed a fine pattern of a release agent
formulation depicting an electronic circuit on the board. In
particular, circuit board 100, made of a conventional composite
material, is laid out on one surface with a geometric pattern 102
over which is sprayed a release agent formulation 108. Release
agent formulation 108 is a finely focused spray emitted from an
artist's airless spray gun 106 to which is supplied the
supercritical fluid - release agent mixture via pressurized system
108, similar to that shown in FIGS. 1 and 2 hereof, and heated line
110. The spray gun may be robotically controlled or controlled by
hand to effect the deposition of the release agent formulation
within pattern 102. The remainder of the board's surface 104 is
left for a masking coating. FIG. 4 shows the dip coating of the
release agent treated board 100 of FIG. 3 into a container 120
containing coating material 122. After dipping board 100 in coating
122, the board may be treated to dry or cure the coating, as
required, and coated board 100 is thereafter wiped according to the
procedure depicted in FIG. 5.
FIG. 5 provides a perspective view of wiping procedure for removing
coating at the release surfaces of a circuit board which had
undergone the treatment characterized in FIGS. 3 and 4, supra. In
FIG. 5, board 100 with coating 134 thereon, is subjected to a
gentle wiping or abrading by rotating roller 132 over the surface
of board 100. Roller 132, rotating counterclockwise in FIG. 5,
contains soft bristles on its cylindrical surface that gently wipes
away the portion of coating 134 located over pattern 102 to which
had been provided the release agent formulation. As a result, the
remaining coated portion of the board's surface are sections such
as 132. This process can be repeated by cleaning the pattern 102
until it is free of the release agent formulation and then printing
a circuit in the pattern. This procedure can be extremely effective
by using a coating that is not receptive to the coating procedure
used for effecting the printed circuit. This same technique can be
used to control the coating of many different kinds of items. For
example, one can use the technique to dip coat a part which is
eventually to be welded to another part. Application of the release
agent to the site for effecting the weld, and then dip coating the
part, followed by a wiping of the part to release the coating over
the release surface, provides the weld surface free of the coating.
In this manner, the part can be coated more extensively than it
could if it were welded first and the welded parts were
collectively dip coated.
FIG. 6 demonstrates the breadth of the invention. In FIG. 6, baking
pan 140 is internally coated via spray gun 142 so that release
agent formulation spray 144 uniformly coats the interior of pan
140. This procedure is amenable to an automated procedure, such as
a robotically controlled spray gun that sweeps the interior with
the release agent formulation. As a result, the application of the
release agent can be effected as part of an assembly line. The
release agent formulation may be a vegetable shortening dissolved
in the supercritical fluid. Supercritical carbon dioxide is a
preferred supercritical fluid for this application because it is
inert to the release agent at the operative temperatures.
A preferred objective of this invention is to demonstrate the use
of supercritical fluids, e.g. supercritical carbon dioxide, as a
solvent in mold release formulations for polyurethane foams. Prior
to the present invention, mold release formulations for
polyurethane foams were of two types, hydrocarbon solvent-based and
water-based. The water-based compositions are not as effective as
the hydrocarbon solvent-based compositions in effecting mold
release. The hydrocarbon solvent-based formulations typically
contain a wax which is dissolved or dispersed in a hydrocarbon
solvent, e.g. a naphtha. The mode of use of these hydrocarbon-based
compositions is to spray the liquid formulation onto the heated
mold surface. Upon contact with the surface, the hydrocarbon
evaporates leaving behind a coating of wax on the mold surface. The
wax layer so deposited, effects the release of the urethane foam
without damage to its integrity and having the appropriate skin
characteristics. The process of the invention provides effective
mold release of polyurethane foam with the minimization of use of
hydrocarbon solvents. The following examples are provided to
further illustrate the advantages of the invention in effecting the
mold release of polyurethane foam. The examples are intended to be
illustrative in nature and are not to be construed as limiting the
scope of the invention.
EXAMPLE 1
This example illustrates the practice of a supercritical fluid mold
release application process in a batch mode. The spray apparatus
shown in FIG. 1 was used for this purpose.
A mold release formulation was prepared from 3,178 grams of a mold
release compound and 3,904 grams of carbon dioxide. The mold
release compound contained 273 grams of a microcrystalline paraffin
wax having a melting point range of 88.degree. to 100.degree. C.
and 2,905 grams of a hydrocarbon solvent having a boiling point
range of 150.degree. to 190.degree. C. The total weight of the mold
release composition was 7,082 grams of which carbon dioxide was
55.1 wt%, the hydrocarbon solvent was 41.0 wt% and the wax was 3.9
wt%.
This composition was prepared as follows:
The 10-liter high pressure vessel 59 was flushed with carbon
dioxide from the high pressure carbon dioxide cylinder 1. With the
10-liter vessel at ambient temperature and pressure, the release
compound was charged to the vessel through charging funnel 71.
Vessel 59 was kept closed from the atmosphere and the carbon
dioxide was charged to it via the CO.sub.2 feed tank 31 using the
Haskel pump 49. The 10-liter vessel 59 was then isolated from the
pump. The pressure in vessel 59 was 850 psig and the temperature
was 20.degree. C. Line 67 to the spray gun 85 was then opened and
the pressure dropped to 700 psig. Spray gun 85 is a Graco.TM.
airless spray gun having a 13 mil orifice in the spray tip with a
60.degree. fan width. The contents of the vessel were heated to
38.degree. C.; the contents of line 67 were maintained at a
temperature between 37.degree. and 40.degree. C. The pressure in
vessel 59 was 2,400 psig. The contents of vessel 59 were then
sprayed onto the internal mold surface of a hot (93.degree. C.)
laboratory mold for a period of 4 seconds. The spray which was
produced was very fine and mistlike. Before spraying with this mold
release composition, the surface of the hot mold was wiped clean of
any residual wax from prior applications. A typical HR (high
resiliency) urethane molded foam formulation, of low water content
(about 3.3 part by weight per hundred parts by weight total of
polyol) was poured into the mold at a mold temperature of
65.degree. C. and the mold was then sealed. At the end of the
customary de-mold time for this formulation (in this case 3
minutes), the lid was opened and the polyurethane foam was found to
release easily and cleanly from the mold. The foam had a good,
smooth surface typical of this formulation.
EXAMPLE 2
The same spraying apparatus, mold release composition, mold, and
urethane foam formulation were used as in Example 1. The contents
of the vessel 59 were at 2,050 psig and 34.degree. C. before
spraying. The flexible hose 67 leading to the spray gun was at
40.degree. C. at the vessel 59 end and 45.degree. C. at the spray
gun 85 end. The contents of vessel 59 were then sprayed for a
duration of 4 seconds onto the mold surface which was at 93.degree.
C. Before application of the mold release composition, the surface
of the hot mold was wiped clean to remove any residual wax. The
foam formulation was poured into the mold which was at 65.degree.
C. and which was then sealed. When the foam was demolded, it
released easily and cleanly. The foam had a good, smooth surface
typical of this formulation.
EXAMPLE 3
The same spraying apparatus, mold release composition, mold and
urethane foam formulation were used as in Example 1. The contents
of vessel 59 were at 950 psig and 33.degree. C. before spraying.
The flexible hose 67 was at 40.degree. C. at the vessel 59 end and
29.degree. C. at the spray gun 85 end. The contents of vessel were
sprayed for 4 seconds on the mold surface which had been cleaned of
residual wax. The mold surface was at 93.degree. C. The spray was
coarser than in Examples 1 and 2. The foam formulation was poured
into the mold whose surface was at 63.degree. C. and then the mold
was sealed. When the foam was demolded, it released easily and
cleanly. The foam had a good, smooth surface typical of this
formulation.
EXAMPLE 4
The same mold and urethane foam composition were used as in Example
1. The mold was heated to 93.degree. C. and wiped clean of residual
wax as before. No mold release composition was sprayed onto the
mold surface. The foam formulation was poured into the mold at
63.degree. C. and the mold was sealed. When the foam was demolded,
it stuck to the mold and tore apart.
EXAMPLE 5
The same mold and urethane foam composition were used as in Example
1. The mold was heated to 93.degree. C. and had no residual wax.
Carbon dioxide only was sprayed onto the mold surface. The foam
formulation was poured into the mold at 65.degree. C. and the mold
was sealed. When the foam was demolded, it stuck to the mold and
tore apart.
EXAMPLE 6
The same spraying apparatus, mold, and urethane foam formulation
were used as in Example 1. A mold release formulation was prepared
from 454 grams of a high-solids mold release compound and 3,632
grams of carbon dioxide. The high-solids release compound contained
77.2 grams of the wax characterized in Example 1 and 376.8 grams of
a hydrocarbon solvent characterized in Example 1. The total weight
of this mold release composition was 4,086 grams of which carbon
dioxide was 88.9 wt%, the hydrocarbon solvent was 9.2 wt% and the
wax was 1.9 wt%.
The composition was prepared at ambient temperature as in Example
1. Vessel 59 containing the composition was then heated so that the
mold release composition was at 1,350 psig and 52.degree. C. The
contents were then sprayed for 4 seconds onto the surface of the
hot (93.degree. C.) mold which had been wiped clean of any residual
wax. The urethane foam formulation of Example 1 was poured into the
mold which was at 65.degree. C. and the mold was sealed. When the
foam was demolded, the foam released easily and cleanly and had a
good, smooth surface typical of this formulation.
EXAMPLE 7
The same spray apparatus and mold were used as in Example 1. A mold
release formulation was prepared from 227 grams of a high-solids
release compound of Example 6 and 4,540 grams of carbon dioxide.
The total weight of this mold release composition was 4,767 grams
of which 95.2 wt% was carbon dioxide, 4.0 wt% was hydrocarbon
solvent and 0.8 wt% was wax.
The mold release composition was prepared at ambient temperature as
in Example 1. It was then heated in vessel 59 so that the
temperature was 50.degree. C. and the pressure was 1,500 psig. The
contents of vessel 59 were then sprayed for 4 seconds onto the
surface of the hot mold (85.degree. C.) which had been wiped clean
of any residual wax. The urethane foam formulation of Example 1 was
poured into the mold which was at 65.degree. C. and the mold was
sealed. When the foam as demolded, the foam release easily and
cleanly and had a good, smooth surface typical of this
formulation.
EXAMPLE 8
The same spraying apparatus and mold release formulation as in
Example 7 were used. Also, a conventional air gun (Speedaire) and a
mold release composition containing 8.6 wt% solids (a mixture of a
microcrystalline paraffin wax having a melting point range of
88.degree. to 100.degree. C. and a hydrocarbon solvent having a
boiling point range of 150.degree. to 190.degree. C.) free of
supercritical carbon dioxide, were used.
The conventional air gun was used to spray the release compound
composition into 12 different containers. Each container was
sprayed into for 4 seconds. The average amount of solids deposited
per container was 4.18 grams with a standard deviation of 1.72.
This corresponds to an average of 44.4 grams of hydrocarbon emitted
during each spray.
The spraying apparatus and mold release formulation of Example 7
(0.8 wt% solids, 4.0 wt% hydrocarbon, 95.2 wt% carbon dioxide) were
used to spray into 12 different containers. Each container was
sprayed into for 4 seconds. The average amount of solids deposited
per container was 0.45 grams with a standard deviation of 0.16.
This corresponds to an average of 2.20 grams of hydrocarbon emitted
during each spray. This average amount represents 5.0% of the
average amount of hydrocarbon emitted during the spraying with the
conventional air gun and the release compound composition.
Therefore, an average of 95.0% reduction in hydrocarbon emissions
was achieved through the use of the mold release formulation used
in Example 7.
EXAMPLE 9
The same spraying apparatus, mold release composition, and mold
were used as in Example 7. The mold release composition was at
50.degree. C. and 1,350 psig when it was sprayed onto the surface
of the hot (87.degree. C.) mold. A urethane foam formulation of
medium water content (about 5.5 parts of water per 100 parts by
weight of total polyol) was poured into the mold at 65.degree. C.
and the mold was sealed. When the foam was demolded, it released
easily and cleanly and had a good, smooth surface typical of this
formulation.
EXAMPLE 10
The same spraying apparatus, mold release composition and mold were
used as in Example 9. The mold release composition was at
46.degree. C. and 1,275 psig when it was sprayed onto the surface
of the hot (87.degree. C.) mold. A urethane foam formulation of
high water content (about 6.5 parts of water per 100 parts by
weight of total polyol) was poured into the mold at 60.degree. C.
and the mold was sealed. When the foam was demolded, it released
easily and cleanly and had a good, smooth surface typical of this
formulation.
EXAMPLE 11
The same spraying apparatus, mold release composition, urethane
foam formulation and mold were used as in Example 7. The mold
release composition was sprayed at 43.degree. C. and 1,100 psig
onto the hot (85.degree. C.) mold surface. The foam formulation was
poured into the mold and the mold was sealed. When the foam was
demolded, it released easily and cleanly and the surface of the
foam had the good, smooth surface typical of this formulation.
EXAMPLE 12
The same spraying apparatus, mold release composition, urethane
foam formulation and mold were used as in Example 7. The mold
release composition was sprayed at 43.degree. C. and 1,000 psig
onto the hot (87.degree. C.) mold surface. The foam formulation was
poured into the mold and the mold was sealed. When the foam was
demolded, it released easily and cleanly and the foam had the good,
smooth surface typical of this formulation.
EXAMPLE 13
The same spraying apparatus, mold release composition, urethane
foam formulation and mold were used as in Example 7. The mold
release composition was sprayed at 41.degree. C. and 800 psig onto
the hot (87.degree. C.) mold surface. The foam formulation was
poured into the mold and the mold was sealed. When the foam was
demolded, it released easily and cleanly and had the good, smooth
surface typical of this formulation.
EXAMPLE 14
The same spraying apparatus, urethane foam formulation and mold
were used as in Example 1. A mold release formulation was prepared
from 22.8 grams of a microcrystalline paraffin wax, 68.2 grams of
an aliphatic hydrocarbon, and 4,540 grams of carbon dioxide. The
wax and the hydrocarbon were charged to vessel 59 as in Example 1.
The total weight of this mold release composition was 4,631 grams,
such that 0.5 wt% was wax, 1.5 wt% was hydrocarbon and 98.0 wt% was
carbon dioxide.
The mold release composition was sprayed at 51.degree. C. and 1,450
psig onto the hot (87.degree. C.) mold which had been cleaned of
residual wax. The urethane foam composition was poured into the
mold at 65.degree. C. and the mold was sealed. When the foam was
demolded, it released easily and cleanly and had a good, smooth
surface typical of this formulation.
EXAMPLE 15
The same urethane foam formulation and mold were used as in Example
14. No mold release composition was applied to the mold which had
been cleaned of residual wax. The urethane foam formulation was
poured into the mold at 65.degree. C. and the mold was sealed. When
the mold was opened, the foam did not release and tore apart.
* * * * *